class SingleDog{
      show(){
        console.log('我是一个单身狗')
      }

}
SingleDog.getInstance=(function(){
    let instance=null
    
    
    return function(){ 
        if(!instance){

        instance=new SingleDog()
    }
        return instance
    }
    
   
})()
const s1=SingleDog.getInstance()
const s2=SingleDog.getInstance()
console.log(s1)
console.log(s2)
console.log(s1===s2)